One method of printing a large number of small printed objects is to print many copies of an original one after the other. Alternatively, a large-size printed object is obtained by printing a large-size original in which the same small printed objects are arranged regularly in a matrix form, and then each of the small printed objects (hereinafter, “fragment”) is cut from the large-size printed object. In the later method, printing inspection is performed to check the printing quality of each of the fragments before cutting the large-size printed object. The printing inspection is performed by using an image of the large-size printed object fixed to a large drum called an inspection drum. Such an inspection drum is included in a printing machine that prints the large-size printed object. While the inspection drum transports the large-size printed object one after the other to the following step, an image of the large-size printed object present on the inspection drum is captured with a line camera.
The printing inspection of a character contained in the color printed object is performed after removing a design present in the background of the character from the color image obtained by capturing the character. By removing the background design, the printing quality of the character can be evaluated accurately without being affected by the presence of any background design.
Various methods are available for removing the background design of the character from the color image of the printed object. For example, a color dropout processing is known in the art. In the color dropout processing, to perform character recognition processing of the characters entered in a form, lines, background patterns, and the like printed previously on the form, i.e., pixels in a color different from the color of the characters, are removed from the color image obtained by capturing the form. For example, Patent Document 1 discloses a method that allows the color dropout processing to be performed accurately. In this method, a processing object and a background pattern in a color image are separated and the background pattern is dropped out by using two gray scale values. One gray scale value is obtained based on a color of the processing object and a color of the background pattern. Another gray scale value is obtained based on a color of an area that does not contain the processing object. Moreover, another method is disclosed in Patent Document 2. In this method, pixels forming a character and pixels other than those pixels forming the character are separated based on a value obtained by performing a linear coupling of an eigenvector set previously and a brightness value of each of the pixels of a color image.
To perform the printing inspection, it is necessary to acquire an image to be used in the printing inspection with a high accuracy. To capture by a line camera a high-accuracy image of each of the large-size printed object transported one after the other on the inspection drum rotating at a high speed, it is necessary to install the line camera in a normal installation position defined with respect to the inspection drum. The installation position is defined by a position and an angle of the line camera with respect to the inspection drum.
Various methods for adjusting the installation position of a camera are known in the art. Patent Document 3 discloses a method for adjusting the installation positions of a plurality of cameras while checking an image obtained by capturing a reference pattern arranged at an imaging position. Specifically, the position of each of the cameras is adjusted so that the reference pattern contained in the image captured by each of the cameras satisfies a predetermined positional relationship. Patent Document 4 discloses a method for adjusting the installation position by capturing with a CCD sensor a line pair chart of a black-white stripe design. An installation position and the like of the CCD sensor are adjusted so that an amplitude of sine wave obtained by capturing the line pair chart becomes the maximum. Specifically, adjustment of an optical system or the adjustment of the installation position of the CCD sensor is performed based on an MTF curve so that the white and black contrast is the maximum.